Telephone ringing circuits

Copyright Tomi Engdahl 1997,2000,2005

Preface

Telephone circuit gain always interrest, because telephones
are everywhere and quite often there are old telephone
luying around somewhere. Those telephones can be used
for many interresting experiments including small
home intercom:
connect telephones in series or parallel and feed suitable operating
current (about 20 mA) to them through resistor from power supply.

The most problematic to home experimenter is how to get
telephone ringing because the ringing voltage is over
50V and not at standard mains frequency (50/60Hz).
Sometimes you want to get the information that telephone is ringing
to your own circuits. This text tries to clear out those problems.

What is ring signal ?

The telephone company sends a ringing signal which is an AC waveform.
Although the common frequency used in the United States is 20 HZ
and in Europe is typically 25 Hz,
it can be any frequency between 15 and 68 Hz. Most of the world
uses frequencies between 20 and 40 Hz. The voltage at the
subscribers end depends upon loop length and number of ringers
attached to the line; it could be between 40 and 150 Volts.
The ringing cadence - the timing of ringing to pause -
varies from telephone company to company.

The usual arrangement is to feed the 75 V a.c. ringing current
(backed by earth) down one wire of the phone line. On the other wire is
placed a slugged relay (or equivalent) which is backed by -48V d.c.
When you pick up the phone, the relay operates to the loop d.c. current
and trips the ringing current. It also triggers a further device to put the
transmission bridge in circuit to enable speech to take place, together
with supervision of the calling and called loops. The ralay needs to
be a slugged relay to prevent premature ring trip by the a.c.
ringing current.

In USA minimum ring voltage supplied is 40Vrms
(delivered into a 5 REN load). This is the must detect limit.
There is also a minimum must ignore value of 10Vrms.
Milage on individual PBX's will vary greatly. But most guarantee to
deliver 40Vrms into a 3 to 5 REN load.

When the telephone ring signal is sen to the telephone, the ring voltage
is not applied constanly to the line. Typically ring timing is
2 seconds on and 4 seconds off in the US. In the UK ring timing goes .4
sec on, .2 sec off, .4 sec on, 2 sec off then repeats. In toher countries
the ring timign cna vary from country to country (even from operator
to operator) and you should check the local regulations if you want
to get to know the actual ring signal timing in use.

What is REN ?

REN stand for Ringer Equivalen Number. It is a measurment of
how mugh ringing power certain telephone equipment takes.
REN numbers are used in USA to determine how many telephoen equipments
you can connect to same telephone line and still get
them ringing properly (typical line can drive about 3-5 REN load).

The definition of 1 REN is the ringer
power required by one ringer of an AT&T standard 500 series telephone
set in single-party configuration (ringer placed ACROSS the line).
One place to find the exact info: get a copy of 47CFR Part 68 - this
is the FCC technical specs (and other info) regarding the PSTN (public
switched telephone network). This info also may be available from the
FCC's web site.

What is ringing tone ?

Ringing tone is the ringing that can be heard while
the receiver is on-hook and somebody tries to call you.
The terms used for describing this telephone ringing are not always
very clear whet they mean, because the same term has been used
in differnet places to mean different things.
ITU-T Q.9 indicates the preferred term is "ringing tone",
but that "ringback tone" is used in the USA. On the other hand, Bellcore
(and the old Bell System), used "audible ringing tone" in many of their
documents. In 5ESS switch documentation (according some news articles),
RINGBACK is used only to describe various ways (other than a normal
terminating call) by which a subscriber's telephone may be rung. Usually
people say "ringback" in place of "ringing tone".

What is distinctive ringing

Distinctive ringing is a system where different ringing tone patterns
can tel different thing about the telephone calls. Typical applications
are PBXs where you can identify if the call is from inside buildign or
from outside by hgearing different ring pattern. Aother applications
are when multiple phone numbers are assigned to one physical line
and the rign pattern tells which number of them has been called.

Distinctive Ringing and Call Waiting patterns and timing use in
USA are covered in GR-506-CORE.
Use of multiple patterns to identify the CALLED party
(multiple DNs per line) is covered in the basic LSSGR (GR-505 and GR-506
in particular),
in the ability to assign ringing patterns to numbers and to Centrex services.
ANSI T1.401 identifies some other requirements for distinctive ringing
involving inter-exchange carriers.

Normal telephone wiring

In normal telephoen wiring (used in Finland, USA and very amny other countries) the
telephoen audio and sing signals share the same wire pair. Typical wiring for
6 pin modular connector:

1
2
3 a-wire
4 b-wire
5
6

A and B wires make the pair which telephone used. Typicslly the modular connectos used in
telephone have only 2 or 4 pins installed. Normally unused pins are used for wiring
more than one line to same connector or for some special applications.

There are also many other types of telephone line connectors in use, but nowadays
this modular connector is the most common in telephone terminal equipmens like
telephones with removable cord, modems and FAX machines.

Special cases in ring signal wiring

On some coutries the ring signal is fed to the customer telephoes
using one extra wire. The UK wiring tetails are available in separate
UK wiring document.

Ringer circuits in telephones

Classical bell type ringer

The most classical telephone ringer circuit is a mechanical bell
controlled by an electronic coil. The circut consists of the
bell coil and a capacitor (usually 470 nF to 2 uF rated for 250V or more)
in series with it. This circuit is connected in parallel to other telephone
electronics. The capacitor in the circuit stops the DC in to pass
through the bell coil, but it lets the ring voltage through easily.
Because of mechanical nature of the ring circuit, it is very sensitive
to the frequency of ring voltage and other than the resonance frequency
of the bell system (usually around 20-25 Hz) do not generate satisfactory ring.

The coil has usually so high impedance that it does not disturb
the telephone audio circuit operation when telephone is off-hook.
Other possiblity is that the ring circuit is disconnected when
the telephone is picked off-hook.

Electronic ringers

The ringer circuits in the modern telephones have the same basic idea,
but the coil controlled bell is replaced by modern electronic ringing
chip and small speaker. The capacitor is still used in series with
ring IC input to make only AC pass to the ring chip. The electronic
ringing circuits are not sensitive to the ringing voltage and they
easily ring with ring signal frequencies between 16 Hz and 60 Hz.

Ring detection circuits in modems

In computer modems the logical signal from ringing is needed instead of
ringing tone. The ring circuit must pass the ring signal information to
modem electronics and still provide electrical isolation between telephone
line and modem electronics. This ring detection is usually done using
one optoisolator circuit, which replaces the raditional ring circuit.
The optoisolator output can be easily connected digital electronics, but
the optoisolator input side needs more electronics: one capacitor
for not letting DC to pass through optoisolator, one resistor to
limit the cirrent passing through optoisolator LED and one reverse
conencted diode in parallel with optoisolator LED to prevent negative
voltages from damaging the LED. This is the basic ring detection circuit.

Usually there is also two zener diodes (usually 10-20V models)
to make sure that the ring detection
circuit does not detect too small AC signals in the line as ring signal.
In the picture below you see a very typical ring detector circuit
for modems. The circuit just gives the idea how modem ring detector circuit
work. The actual component component values selection must be so that
the circuit meets the national telephone regulations (this can be
usually easily done by using suitable zener diodes and maybe chancing
the resistor value a little).

NOTE: You can get the circuit work by taking out D1 and D2 and
replacing them with a short circuit. The circuit works after then,
but it is possible that in this case some low voltage noise on the
line can cause the circuit to ring. Different countries have different
specifications on how low voltages should not cause a telephone to
ring at all.

Another apprach for ring detecting is to use a full wave rectifier
circuit to convert the AC sign signal to the DC suitable for optoisolator
and then put current limiting resistor and zener diode to the rectifier
output.

Other ideas to detect telephone ringing

One idea which is proposed in many sources is to use small neon bulb
(like those used as lights in some mains switches) for detecting the
ring signal. The circcuit proposed is to connect one neon bulb and
47kohm resistors in series and connect this to telephone line.
The neon bulb has about 60V trigger voltage to start conducting, so standard
48V telephone battery voltage does not light it. When the AC ring signal
is added to that voltage, the voltage is enough to light the neon bulb.
The neon bulb can be used as visual indicator or electronics can sense
it with LDR photoresistor or phototransistor.

If you don't want to build your own circuit from neon bulb and resistor,
there is
an even easier solution is to go down to the hardware store and get a
"pigtail" tester. It has two nice leads that one normally pokes into the
wall outlet to test for voltage. Wire it instead to the phone line. This
saves the hassle of trying to find the container for the neon lamp, and
the resistor (which is VERY necessary, take my word for it).

One modem schematic I have seen used quite special method for detecting
ringing signals: It had a small capacitor in parallel with on-hook/off-hook
control relay contacts. This capacitor let some small part of the sound
and ring signals pass to the telephone transformer. In this way those
ring signals can be detected as small signal pulses in transformer
secondary (and this circuit can be also used for Caller ID signal detection).
The capacitor was so small that the impedance seen from telephone line
stays high enough not to disturb other equipments in the same telephone line
when modem is no on-line.

What telephone regulations say about telephone ringers

European NET4 telephone line terminal equipment specs define the following
specs for the telephoen ringing detector circuit.

The impedance in voice frequency (200-3400 Hz) must be greater than 10 kohm when measured with 0.5V RMS audio signal

The current taken by the ringer must be equal or less than 5 mA at 35 V ring voltage and equal or ledd than 10.7 mA at 75V ring voltage. The measurments are made using 25 Hz ring current frequnecy.

Ring detector must work on ring signal which is 44-58V DC summed with 25+-3Hz AC ring signal in voltage range 35-75 V. The feeding resistance for ring generator is 800-1710 Hz.

Ring detector must not detect ring signal which is 44-58V DC summed with 20-3400 Hz AC ring signal which is less than 10 V. The feeding resistance for ring generator is 800-1710 Hz.

If the equipment is automatically responding the equipment must wait at least 1s from
the ring detection until it goes off-hook.

Telephone ringer classification

In USA FCC regulations need the ringer type to be specified on the
device. The possible types are Class A and Class B.
Class B ringers will respond to ringing frequencies of between 17 and 68 Hertz while Class A ringers will respond to betwwen 16 and 33 Hertz.
Class A devices are those typical old telephone bells and
practically all electronic ringers are B type.
Nearly all of the devices made to connect to the phone lines today are of the Class B type.
The telephone ringer type on your device (if you live in USA)
is printed on the FCC sticker on the bottom with a REN number on it.
You'll see something like .9B (= REN 0.9 Class B) or 1.0A (= REN 1.0 Class A).

How to make telephone ring

The following ideas are simple circuits, which generate
ringing voltage at mains frequency (50 or 60Hz depending on country).
They will ring modern telephones very well, but the rign sound might not
be actually the same as with right ringing signal. If that is
not a problem, then go on. The ring signal at 50 or 60 Hz does not work
with old telephones which have mechanical bells in them.

Direct connection to mains

This approach has been proposed many times at rec.theatre.stagescraft
newsgroups but I strongly suggest not to use it. Mains voltage (120V AC 60Hz)
used in USA makes the modern telephones ring, but it is dangerous to make
direct connection to mains voltage. And if you don't use any type
of current limiting, the telephone will cause dangerous short circuit
when it is picked up. The telephone will destroy and put out smoke.

50/60Hz ring voltage generated from mains voltage

If you want to use very simple circuit for ringing, I would
suggest following combination: a small ready made AC adapter
which puts out AC and a small transformer connected to it.
If you use suitable tranformer combination, you will get
nice 70-90V AC voltage at you mains voltage frequency (50 or 60 Hz).
Ready made wall adapter will provide provide isolation from
mains voltage and also limit the current in short circuit
situation.

Suitable comination for example is wall adapter which outputs 8-9V AC
at 200-500 mA connected to transformer which has 120V primary, 12V secondary
and power handling capacity of few watts.
The wall adapter is connected to transformer's
12V secondary through a button. When the button is pressed, there
is about 70-90V AC available at transformer's primary winding.
For current limiting it is a good idea to put 1 kohm
3W resistor in series with transformer's secondary. If you can't find
transformer I told earlier, remeber that many transformers with
220V primary winding have center tap connection for 110V voltage wiring.
And if there is not centre tapped 220V transformer, you can always use
220V to 24V transformer. If your wall transformer has different rating,
the scale the transformer's values according that. The component
values in this circuit are not critical, but keep in mind that the voltage of
transformer's secondary must be grater thatn the output voltage of
the wall adapter.

And for your safety,
build this circuit to a good box in which you have telephone connector
on one side. And be careful with the circuit not to get shocked because
the 50/60Hz ring voltage is more dangerous than normal ring voltage.

Methods for generating good ring voltage

Ringer module

The easiest way to get real ringing module. Those units are available from
some companies which make DC/DC converters for telecommunication industry.
Migh not be the easiest component to get.

70V line PA amplifier

The output voltage of PA amplifiers designed for driving 70V speaker
system speakers have enough output voltage and power for ringing
telephones. If you have old this type of amplifier lying
somewhere, you can connect the amplifier input to fuction generator
and output to telephone through 1 kohm 3W resistor. When you set
the function generator to generate sine wave at 20-25 Hz at suitable level
for amplifier, you have an adjustable level ring generator.
Usually those amplifiers are not good at playing back frequencies
below 50 Hz, so you might have to try higher frequencies if that does
not work as expected.

Normal audio amplifier and transformer

Very nice variable amplitude ring generator can be built from
audio amplifier designed for driwing 4 or 8 ohm speakers and have
output power of 3W or more, 10 ohm 10 W resistor, 220V to 12V transformer
(few watts), 1000 ohm 3W resistor and function generator.

The circuit is easy to build. Connect 10 ohm resistor in series with
transformer's secondary winding and 1000 ohm resistor in series with
primary winding. Connect the primary winding side of the transformer to
amplifier's speaker output. Connect the telephone to the secondary side.
The resistors are in the circuit to limit the current and to keep
the impedance high enough for the amplifier.

When you have done this, connect you function generator to amplifier's
input and set it to generate 20-25 Hz sine wave at suitable level
for amplifier's input. Turn down the volume of the amplifer. Turn
the amplifier on. Turn the volume up until you hear telephone
ringing well. You can check the ringing voltage with multimeter
if you vat to make it to exactly right level.

Modified power inverter circuit

It is possible to make 17 - 25Hz a.c. from d.c. A simple multivibrator will
do it. You then need a power transistor or similar to give the high-current
output. A suitable circuit can be modified from typical
power inverter
circuit by changing the timing components to make the frequency
to 20-25 Hz range. Then the transformer needs to be selected so
that it matches this application (for 12V operation take a
mains centre-tapped 60V (30+30V) secondary and 230V primary).

Dedicated ringing generator circuit

There have been telephone ringer circuit in major electronics magazines
and circuit books. Those circuit are good idea when you want to build
the circuit from base components.

There are commercial units specifically made fro ringing telephone.
TELE-Q is a device
designed for ringing telephone theatre effect. That unit is available
from Norcostco
for little over 100 US dollars. I have no experience in this product but it
has been suggested in many usenet news articles.

Maplin Electronics has a phone ringer
electronics kit which can give out UK and USA type ring styles.
It has been reported to work uite well with any modern telephone, though
it has said to struggle slightly to drive old fashioned bell types
which need lots of ring current.

There are also telephone line simulators available from some
tecom equipment manufacturers. Those telephone line simulator
boxes also usually include the ringer circuit. Two examples
are Viking Electronics Line Simulator/Ringdown Circuit and Jech Tech Phone Helper.
Usually complete line simulators are more expensive
than simple ringer circuit but they have more uses also (you can make
two telephones an intercom etc.).

Generating ring pattern

Normal telephone ringing signal the central office sends is not normally
contirnuous signal, but follows some pattern.
The pattern could be for example ring 2 seconds on, four seconds off
and then again 2 seconds on, 4 second off etc..
The patterns used can vary somewhat from country to country.

If you want to generate this kind of pattern you need
a timer circuit that generates 2 seconds on and 4 secodns
off type output signal. That signal is then used to control
a relay that switches the power from the power source going
to telephone and off.
A 555 timer and one relay can nicely do this.
Basicly you take 555 timer in normal astable mode and
then select the value fo two resistors and one capacitor.
Then connect relay to 555 output, and that should do it.